For the production of corrugated cardboard, plants comprising one or more single facers are used, to each of which two smooth cardboard sheets are fed. A first smooth cardboard sheet is corrugated in the single facer between a first corrugating roller and a second corrugating roller. The first, corrugated cardboard sheet is glued to the second, smooth cardboard sheet by means of a pressure device, for example a belt or a roller. In this way a so-called single-faced corrugated cardboard, composed of a corrugated sheet and a smooth sheet, also called cover, can be obtained. The single-faced corrugated cardboard is fed, together with a third smooth cardboard sheet, to a series of hot plates where the single-faced corrugated cardboard is glued to the third smooth cardboard sheet, so as to form a second cover.
In some plants more single facers are provided, each of which produces a single-faced corrugated cardboard. Several single-faced corrugated cardboards are simultaneously fed to the series of hot plates, together with the third smooth cardboard sheet, in order to form a composite corrugated cardboard. The composite corrugated cardboard in this case will be constituted by two outer covers and a plurality of corrugated sheets interposed between the covers. Adjacent corrugated sheets are separated from one another by an intermediate smooth sheet.
The corrugated cardboard sheet coming out from the series of hot plates is subsequently trimmed, eliminating the longitudinal trimmings, and divided in longitudinal strips, each of which can be suitably scored. The strips are subsequently transversely cut to form corrugated cardboard sheets with score lines, for the production of boxes or other similar articles.
A plant of this type for the production of corrugated cardboard is described for example in U.S. Pat. No. 5,437,749.
To detect any defects of the corrugated cardboard several systems have been studied, particularly based on ultrasonic sensors. Devices for the detection of gluing defects are described in U.S. Pat. No. 7,921,892 and in DE 19955917.
Ultrasonic systems have proved to be not very efficient, and are not able to differentiate among different types of gluing defects.
JP-A-1267404 discloses a system for detecting gluing defects on corrugated cardboard, wherein an optical system is used; this optical system having an illuminator generating a light line oriented transversally relative to the crests of the single-faced corrugated cardboard, and a video camera. The defects are detected as alteration of the detected image with respect to a theoretical corrugated line.
U.S. Pat. No. 5,663,656 discloses a device for detecting characteristics of a corrugated cardboard by means of an optical device. The device detects the intensity of a characteristic of the corrugated cardboard and identifies its oscillating frequency.
WO-A-2005/087460 discloses an optical system for detecting the amount of glue applied onto the crests of the corrugated cardboard sheet before coupling thereof to a smooth cardboard sheet. This system is not suitable for the detection of gluing defect.
The known systems and methods have limited functionalities and do not allow an automatic management of the production line.
A need therefore exists, for corrugated cardboard production plants, which are provided with more efficient systems for the detection of gluing defects, particularly on the single-faced corrugated cardboard produced by the single facer.